When will you be releasing the C751A train with interior, really hope to see it soon!

It will be released when it is completed. There won't be any developer-only release, but there may be releases for internal testing, of whom I will recruit at a later date. I'm estimating a release during or before June.
Not much updates for the add-on. If you are on Facebook, I just created a page for the openBVE section of my site - I'll be posting frequent updates there.